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Lateral Growth In Trees: Lateral growth in trees frequently begins later than growth in height, but the period of lateral growth is longer. Rates of lateral growth likewise are low at the outset, increasing after a few weeks and then diminishing. In view of the long annual period of lateral growth in trees, deleterious environmental conditions may exercise a pronounced effect on growth. In the wood of trees of temperate regions, narrow and wide growth rings may be observed, marking years characterized by unfavorable and favorable conditions for growth.
Growth Cycles.—Longitudinal and lateral (diametral) growth do not proceed continuously, but rather are cyclical phenomena with periods of growth activity alternating with periods of relative inactivity. One of the best illustrations of the periodic nature of growth in large woody stems is to be found in the annual ring pattern which characterizes the wood of trees of temperate regions. It has been suggested recently that even in the apparently ever-growing trees of the tropical rain forest, cambial growth is not a continuous process.
Annual Growth.—An individual tree located in a favorable situation grows relatively slowly as a seedling, and rather faster as a sapling and young tree. However, upon reaching maturity, the rate of annual growth decreases to a low level. This pattern of behavior is well illustrated in the decreasing width of the annual growth rings of mature and old trees. There is relative uniformity in the over-all patterns of growth of various tree species, although the actual patterns may differ widely in individual trees of different species. For example, individual Sitka spruce trees in the Pacific Northwest may continue to grow rapidly in height and diameter at 150 years of age, whereas red spruce trees in northeastern North America have reached maturity in less than 100 years, and will grow a negligible amount in subsequent years.
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